Microlithography is used for producing microstructured components, for example integrated circuits or LCDs. The microlithographic process is carried out in a projection exposure apparatus which has an illumination device and a projection lens. The image of a mask (=reticle) illuminated by the illumination device is projected here by the projection lens onto a substrate (e.g. a silicon wafer) which is coated with a light-sensitive layer (photoresist) and is arranged in the image plane of the projection lens in order to transfer the mask structure onto the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process because of the lack of availability of suitable transparent refractive materials.
The operation of, inter alia, mirrors under grazing incidence is known. Here and in the following, mirrors which are operated under grazing incidence and whose use is fundamentally desirable because of the comparatively high reflectivities which can be achieved (for example 80% and more) are mirrors for which the reflection angles based on the respective surface normal which are obtained in the reflection of EUV radiation are at least 65°. Such mirrors are sometimes also referred to as GI mirrors (“grazing incidence”).
To optimize the performance of a projection exposure apparatus, it is not only necessary to make a suitable choice of the respective mirrors or layer materials with a view to desirable optical properties, but also to take account of the fact that impairment of these optical properties (in particular in the form of reflection losses or undesirable changes in the reflection behavior as a function of the angle of incidence) can occur as a result of contamination during operation of the projection exposure apparatus.
With regard to the prior art, reference will be made, merely by way of example, to US 2005/0279951 A1.